1. Field of the Invention
The present invention relates to solid state electronics and, more particularly, solid state electronic devices which use a single electron to convey information.
2. Description of the Related Art
It is possible with modern fabrication techniques to construct capacitors so small that the charge of a single electron on one of the plates of the capacitor results in a measurable voltage being developed across the terminals of the capacitor. The smaller the capacitor, the larger the voltage. Current fabrication limits put the capacitor size at about 10.times.10 nm square, resulting in operating temperatures of 4K.
Since the presence of a single electron on a very small capacitor causes a finite voltage to develop across its terminals, it follows that a finite voltage, e/2C (where e is the charge of a single electron, and C the capacitance of the capacitor) must be applied to the capacitor before the first electron will move onto the capacitor plates. This is referred to as "coulomb blockade". Because of the coulomb effects of a single electron on a tiny capacitor, current flow is blocked until a certain threshold voltage is reached.
Fullerenes are large pure carbon clusters including an even number of 32 or more carbon atoms. They were first experimentally discovered in 1985. See Rohlfing et al., J. Chem. Phys. 81, 3322 (1984) and Kroto et al. Nature, 318 162 (1985). The most stable fullerene is believed to be a fullerene containing sixty carbon atoms ("C.sub.60 ") which is believed to have a structure like a soccer ball, i.e., a hollow sphere made up of 20 hexagons and 12 pentagons, arranged as a truncated icosahedron. A carbon atom is located at each vertex of the hexagons and pentagons. Gram size quantities of C.sub.60 have been manufactured. See Kratschmer et al., Nature, 347, 354 (1990). C.sub.60 is always exactly the same size. C.sub.70 has also been observed in large quantities. It is believed that C.sub.70 has a structure like a football. The diameter of C.sub.60 is about 1 nm. See Heiney et al., Phys. Rev. Lett., 66, 2911 (1991). Both C.sub. 60 and C.sub.70 can be made electrically conductive by alkali-metal-doping. See Haddon et al., Nature 350, 320 (1991). C.sub.60 becomes superconductive when doped with potassium (See Hebard et al., Nature, 350, 600 (1991)) or rubidium (See McCauley et al., J. Am. Chem. Soc., 113, 8537 (1991).
The hollow nature of fullerenes makes it possible to trap atoms. In other words, atoms are trapped inside of carbon cages. This has been demonstrated with lanthanum atoms. One to four lanthanum atoms have been trapped inside varying size fullerenes. See Baum, Chem. & Eng. News, pg. 5, October 1991. Significant modification of the characteristics of fullerenes can be obtained by trapping atoms. Atoms can also be attached to the outside of fullerenes.
Conventional single electron devices utilize small metal balls or small regions of two dimensional electron gas confined by electrostatic forces. Both techniques, however, are not satisfactory. The smallest metal balls used to date have been approximately 10 nm in size. These 10 nm balls develop good signals at 4K but are not desirable for room temperature operation. The smallest two dimensional electron gas regions to date require operation at or below 1K.
Moreover, the metal balls and the regions are all slightly different in size due to fabrication variances. Current fabrication techniques, cannot obtain the size uniformity required for the use of these devices in large scale integrated circuits.